1. Field of the Invention
The invention relates to a sigma-delta modulator, and more particularly, to a continuous-time sigma-delta modulator with excess loop delay compensation.
2. Description of the Related Art
Generally, continuous-time sigma-delta modulators are sensitive to delays in feedback paths. The delays are introduced by a quantizer or any other circuits processing digital output signals, called excess loop delays (ELDs). A delay causes instability of the continuous-time sigma-delta modulators. In prior arts, the effect of the excess loop delay in a continuous-time sigma-delta modulator is compensated with the introduction of a constant term to the transfer function through at least one additional feedback digital-to-analog converter. Depending on the position where the delay compensation is applied, an additional operational amplifier is required for the feedback digital-to-analog converter, or two feedback digital-to-analog converters are required, which increases the area and power consumption of the continuous-time sigma-delta modulator. When the position where the delay compensation is applied is located before a quantizer, an additional pole induced by the feedback digital-to-analog converter may degrade the loop stability. In some other cases, when the position where the delay compensation is applied is at two input terminals of an operational amplifier, the transfer function of the continuous-time sigma-delta modulator may be changed which is disadvantageous to circuit designs.